Catalysts for the oxidation of unsaturated aldehydes to unsaturated acids are generally well known in the literature and in various patents. U.S. Pat. Nos. 3,865,873 and 3,882,047 and Japanese Pat. No. 47-33082 disclose such catalysts wherein ammonia or an ammonium-containing compound is incorporated in the preparation of the catalysts.
U.S. Pat. No. 2,865,873 in Column 13, Examples 101 to 104 discloses a process for the preparation of methacrylic acid using catalysts consisting of molybdenum, phosphorus, titanium and oxygen, wherein ammonium para-molybdate is employed in the preparation of the catalysts. The highest yield of methacrylic acid produced is about 39.56%.
U.S. Pat. No. 3,882,047 discloses the preparation of methacrylic acid using catalysts containing molybdenum, phosphorus, at least one element such as thallium, rubidium, cesium and potassium, and at least one element such as chromium, silicon, aluminum, iron and titanium. This reference teaches the incorporation of ammonia or ammonium-containing compounds in the preparation of catalysts exemplified in the oxidation of methacrolein or acrolein; phosphomolybdic acid is employed in the preparation of virtually all catalysts exemplified; and in a few examples, ammonium molybdate is employed. This patent discloses in column 3, lines 30-40 as follows:
"It is preferred that the catalyst be prepared so that the constituent elements will form complex compounds such as heteropolyacids, then acid salts or ammonium salts."
Japanese Pat. No. 47-33082 discloses a process for reclaiming an ammonia-modified phosphorous-molybdenum-X-oxygen catalyst, wherein X is at least one element selected from the group consisting of antimony, arsenic, bismuth, cadmium, germanium, indium, iron, lead, silicon, thallium, tin and tungsten. Preparation of the catalyst involves treating the catalyst with the ammonia and water by oxidizing the catalyst in advance or by oxidizing it simultaneously with the treatment of ammonia and water. This patent discloses that the ammonia forms a complex compound with the other elements present.
Preparation of phosphomolybdic acid based catalyst in the absence of ammonia or ammonia-containing compound is described in U.S. Pat. No. 4,136,110, commonly owned by the Assignee of record herein. However, the process set forth therein is also directed toward catalyst preparation from molybdenum trioxide. Thus, the prior art of which we are aware has not set forth a method by which a phosphomolybdic acid based catalyst can be prepared in aqueous media from phosphomolybdic acid in the absence of ammonia or other basic compound.
Copending application Ser. No. 222,821 by Grasselli et al., assigned by our common Assignee herein, is directed to the preparation of first stage oxide catalysts containing molybdenum and one of bismuth or tellurium in an organic liquid preferably admixed with 5 to 35% water.
U.S. Pat. Nos. 3,959,182 and 4,035,417 disclose the preparation of molybdenum vanadate catalysts having a molybdenum oxide to vanadium oxide weight ratio of 2:1 to 8:1 in an aqueous solution to which is added various organic reducing agents. The liquid media exemplified contain between 4 to 8% organics by weight.
U.S. Pat. No. 4,000,088 discloses the aqueous preparation of phosphomolybdic acid based catalysts utilizing a solution of catalyst components, that upon mixing with aqueous 28% ammonia water, results in a suspension.
Thus it can be seen that the prior art is characterized by the utilization of ammonia or an ammonia-containing compound for the preparation of phosphomolybdic acid based catalysts. It is believed that treatment with ammonia has been employed to react with groups in the catalyst which would otherwise undergo crosslinking during drying and early calcination, a process which significantly deactivates the catalyst before it is even used. Preparation of phosphomolybdic acid based catalysts in alcohol is believed to provide protection for more of these groups than when an aqueous ammoniation process is employed thereby providing a higher activity of the phosphomolybdic acid based catalyst.